Multiple piston oil well stimulation and service devices, such as mud pumps or intensifiers and detensifiers are known. Two ram duplex pumping apparatuses are shown in the patents of Hall, et al, U.S. Pat. No. 3,773,438 of Nov. 20, 1973 and U.S. Pat. No. 3,967,542 of July 6, 1976, and a control for a duplex pump is shown in Hall, U.S. Pat. No. 3,981,622 of Sept. 22, 1976.
A characteristic of a typical oil well high-pressure pumping unit resides in the fact that when one of the ram drive cylinders has completed its forward or working stroke, the very high pressure hydraulic fluid behind the driving piston is released or dumped into the reservoir or tank. While the resultant loss of energy may not seem to be significant when compared to the total amount of energy expended by these large units, the fact is that it can amount to about 2% of the total energy being used, and this can be significant in self-contained systems in which the hydraulic fluid operates through a closed path. In such closed hydraulic systems the unspent or unused energy in the form of heat must be removed from the hydraulic fluid to prevent the same from overheating.
The oil well pumps of the general kind described are commonly operated in regions in which the ambient temperatures may be rather high. Thus, a self-contained system must contain sufficient cooling apparatus and capacity to prevent the hydraulic fluid from exceeding a predetermined maximum temperature. Above this temperature the seals and other parts, as well as the oil, are subject to rapid deterioration. Generally, the desired operating temperature is considered to be about 150.degree. F. The life of the seals is particularly critical, since any kind of operation which shortens or reduces the life of the hydraulic seals substantially increases the likelihood of a premature overhaul or seal change, an expensive and time consuming operation during which time the equipment is idle. The problem of lost energy in the form of heat is further aggrevated by the fact that, in most locations, cooling water is not available, and expensive air operated heat exchangers must be used. The differential temperature, for the purpose of cooling, during mid-day operation, can be rather narrow.